Fix graceful process shutdown in macOS app

[AlexCheema]

Motivation

Fixes #1370

When the macOS app stops exo, GPU/system memory isn't released. This happens because:

1. The macOS app calls process.terminate() (SIGTERM) but the Python process only registers a graceful shutdown handler for SIGINT, not SIGTERM. SIGTERM's default Python behavior raises SystemExit which bypasses the cleanup cascade (runner subprocess MLX cleanup via mx.clear_cache(), channel closing, etc.).
2. The app doesn't wait for the process to actually finish cleanup — it immediately nils out the process reference.

Changes

src/exo/main.py: Register SIGTERM handler alongside SIGINT so the graceful shutdown cascade (Node.shutdown() → cancel task group → worker/runner cleanup → mx.clear_cache() + gc.collect()) runs regardless of which signal is received.

app/EXO/EXO/ExoProcessController.swift: Replace immediate process.terminate() with escalating shutdown per @Evanev7's suggestion:

1. Send SIGINT via process.interrupt() — triggers the registered Python handler for graceful cleanup
2. Wait up to 5 seconds for the process to exit
3. If still running, escalate to SIGTERM via process.terminate()
4. Wait up to 3 seconds
5. If still running, force kill via SIGKILL

The escalation runs in a detached Task so the UI updates immediately (status → stopped) without blocking.

Why It Works

The root cause is that SIGTERM wasn't triggering the graceful shutdown path. By registering a SIGTERM handler in Python and sending SIGINT first from the macOS app, the process gets a chance to run the full cleanup cascade: cancelling the task group, shutting down runners (which call del model; mx.clear_cache(); gc.collect()), closing channels, and flushing logs. The escalation to SIGTERM and SIGKILL ensures the process always terminates even if graceful shutdown hangs.

Test Plan

Manual Testing

• Start exo via macOS app, load a model, run inference
• Stop via the toggle switch, verify memory is released without requiring a system restart
• Test rapid stop/start (restart) to ensure no race conditions

Automated Testing

• uv run basedpyright — 0 errors
• uv run ruff check — passes
• nix fmt — no changes

[AlexCheema]

Hey @Evanev7 can you check this? Does it make sense to also catch SIGTERM like this?

[Evanev7]

yeah catching sigterm is fine